The present invention relates to an improved drum type shear, such as a crop or trimming shear disposed between a rough rolling mill group and a finish rolling mill for shearing off faulty or incomplete end portions, for example such end portions at the leading and trailing edges, of a rolled workpiece to prevent damage in the finish rolling mill in a hot strip mill installation.
In a hot strip mill installation, it has been common practice, for the purpose of preventing damage in a finish rolling mill, that just prior to finishing a rolled workpiece, for example a sheet bar, the leading and trailing ends of the workpiece are regularly sheared at right angles to the direction of travel of the workpiece along a working line by means of a traveling shear, to remove such faulty or incomplete end portions. As such traveling shear, a drum type shear has been commonly employed.
Such a drum type shear is shown in FIGS. 1 and 2, which illustrate a workpiece sheet bar 1 traveling in the direction indicated by the arrow. The workpiece is bitten between an upper blade 3 and a lower blade 5 detachably mounted on a rotating upper drum 2 and a rotating lower drum 4, respectively. By such action there is sheared from each of the leading and trailing ends of the workpiece 1 a crop portion or end cut plate 1'. The prior art drum shear of this type provides that the upper and lower drums 2 and 4 rotate at the same rotational angular velocity, and that the radial distance r.sub.1, between the rotational axis of drum 2 and the outer edge of blade 3, is equal to the radial distance r.sub.2, between the rotational axis of drum 4 and the outer edge of blade 5.
In recent times however, shearing conditions have become more severe such that the workpiece temperature is lowered. This is due to the fact that there is a trend towards shortening the length of a sheared crop portion from approximately 200-300 mm to approximately 30-100 mm, for the contemplated purpose of lowering costs by increasing the rolling yield, and that direct rolling is employed for the purpose of saving energy. As a result, occurrences of accidents and damage have become more frequent in that the crop portion 1' cannot be sheared off perfectly, but sometimes enters the finish rolling mill with the workpiece 1, with the result that the rolls of the rolling mill are damaged or the workpiece is scratched, thereby lowering the commercial value of the product.
It has been discovered that the occurrence of the above-mentioned type of faulty shearing arises more frequently with respect to a tail cut, i.e. shearing of the trailing end, then with respect to a top cut, i.e. shearing of a leading end. This phenomenon is illustrated in FIGS. 3(a) and 3(b ) which show the conditions just prior to completion of shearing, the condition of a top cut being illustrated in FIG. 3(a), and the condition of a tail cut being illustrated in FIG. 3(b). These figures make it apparent that when carrying out a shearing operation by means of a drum type shear, a workpiece 1 is subjected both to a shearing force and to a bending force by the actions of upper blade 3 and lower blade 5. As a result, the cropped portion 1' is bent at a particular angle with respect to the direction of travel of the workpiece along the working line while it is being sheared. In the case of the top cut, as illustrated in FIG. 3(a), a left surface 6 of lower blade 5 is inclined leftwardly by an angle .beta..sub.1 with respect to a verticle plane, and thereby the lower blade 5 to some degree acts to reduce or counteract the bending force imparted to the crop portion 1'. On the other hand, in the case of the tail cut, as illustrated in FIG. 3(b), the right surface 7 of upper blade 3 is inclined leftwardly by an angle .beta..sub.1 with respect to a vertical plane, and therefore the upper blade 3 has much less of an effect of preventing or counteracting the bending force imparted to crop portion 1'. It will be apparent from a comparison of FIGS. 3(a) and 3(b) that the angle .alpha..sub.2 at which the trailing end crop portion is bent will be substantially larger than the angle .alpha..sub.1 at which the leading end crop portion is bent, during respective shearing operations. From the results of practical use on an actual machine, as well as from experiments, it has been determined that, all other operating conditions being equal, the larger is the bending angle .alpha. of the respective crop portion with regard to the direction of movement of the workpiece, the higher will be the occurrence of faulty or incomplete shearing.